A recent development in the field of illumination relates to colour control and brightness control of light sources. Although this technology can in principle be applied to several types of light sources, LEDs are mostly used for this purpose. Since colour control and brightness control of LEDs is known per se, only a brief explanation will suffice here.
LEDs have been developed for generating single-colour light. With a light source comprising three LEDs of mutually different colours, it is possible to generate a light mixture having a colour point in a colour space within a triangle of which the corner points are defined by the individual LED colours. The location of this colour point can be varied by varying the relative average intensities of the individual light contributions. The brightness can be varied by varying the intensities of the individual light contributions to the same extent, keeping the relative average intensities constant. It is noted that one light source may actually comprise multiple LEDs of one and the same colour in order to increase the light output for that colour. A light source of this type comprises in general a driver having an input for receiving a control signal, indicating the required colour and brightness, and having outputs for driving the individual LEDs. On the basis of the received control signal, such driver determines how to drive the individual light sources.
Basically, colour control and brightness control of a light source relies on intensity control of the constituting LEDs. Therefore, in the following the present invention will be specifically explained for intensity control of LEDs, keeping in mind that the present invention is more generally applicable to intensity control of light sources.
Stated briefly, an LED is driven by causing an electric current to flow through the LED. It is possible to use a voltage source but, in view of the fact that light output is proportional to current, it is more suitable to use a current source. The most straightforward manner for varying the light output would be to vary the current level. Changing the current level, however, is not suitable, if only for reason that the output colour may depend on the current level. Therefore, it is common practice to apply duty cycle control. In such case, the LED is switched ON and OFF at a certain switching frequency. In the OFF state, the LED current is zero or almost zero, and the LED produces no light or at least substantially no light. In the ON state, the LED current is maintained substantially constant, thus the light output is maintained substantially constant. The repetition period of the ON/OFF pattern is indicated as current period. The ratio of the ON-duration and the current period is indicated as the duty cycle. The duty cycle determines the average lamp current and hence the average light output. The switching frequency is a design parameter, which should not be chosen too low in order to avoid visible flicker, and which also should not be chosen too high in order to avoid too much switching losses, while further the switching frequency should be within the bandwidth of the driver/LED combination. It is noted that, keeping the lamp current at a fixed level, the average light output is at a maximum when the duty cycle is equal to 1 (i.e. 100%).
In a more recent development, an illumination system comprises a plurality of LEDs distributed over a space, such as a room. It is possible that all LEDs are driven in the same manner, such that the colour and brightness conditions are the same all over the room. However, it would be desirable to be able to independently set the colour and brightness conditions in different sections of the room. A control system for such illumination system may comprise a light sensor, that can be positioned at a certain location, generating a signal that represents the local colour and brightness conditions, and a central controller may amend the control signals for the LEDs of the illumination system such that the required local colour and brightness conditions at the location of the sensor are met. For such operation, the control system needs to know which LEDs contribute to the illumination at that location, and to which extent. In order to be able to determine the identity of the LEDs contributing to the illumination at a sensor location, it would be desirable that the light output of a specific LED contains a code identifying that specific LED.
Incorporating such code in the light output can be done by amplitude modulation of the LED current of the ON state but, as mentioned before, it is preferred that the LED current in the ON state is maintained constant.
Another method for incorporating such code in the light output is switching the LEDs ON and OFF in a certain pattern, the pattern defining a series of “0”s and “1”s of a binary code. Now, the problem arises how the duty cycle switching can be suitably combined with the coded switching, especially in such a way that the code does not interfere with the illumination function.
In an illumination system as described above, i.e. comprising a plurality of LEDs distributed over a space, it is possible that the spacing between LEDs is such that a sensor would only receive light from one LED at a time. However, it is also possible, and in cases with substantially homogenous illumination it is even very likely, that a single sensor (photo detector) would receive light from two or more LEDs simultaneously. In the case of coded switching, the sensor would receive two or more coded signals at the same time, that would interfere with each other. Thus, the problem arises of performing the coded switching in such a way that individual signals from individual LEDs can be reliably distinguished.
On the other hand, the present invention does not necessarily relate to a plurality of LEDs. Even in an illumination system comprising only one single LED, it may be desirable to combine duty cycle control for colour and/or brightness variations with coded switching for data communication. Apart from transmitting an identifying code, it is desirable to use the illumination in a room for data communication to one (or more) user(s) in the room. For instance, the data communicated in this way may contain audio and/or video information. For such applications, a very high data rate would be desirable.